N.I.B. Tone?

Set your amp (or headphones) at a moderate volume. 

Things might start getting 'kinda loud for a little while. 

With the volume & tone controls on your guitar turned all 

the way up, the best way to get Tony Iommi's sound is to turn 

the gain control on the Plexi Pedal to about the 3 o'clock position, 

and then bring the volume control up to wherever you want it. 

Set the tone control on the pedal to the 12 o'clock position 

to start with, and then adjust it as you wish.

Back in the 1960s & 70s, alot of guitarists used smaller amps 

in the studio, and they would turn them up 'all the way' so that 

the distortion was guaranteed. 

Nowadays, you can simply turn the 'gain' or 'pre' control of nearly 

any guitar amp up to ten, and then "very" slowly start turning up 

the volume control. 

BTW - Geezer was using a wah-wah pedal with the bass in the very 

beginning of N.I.B.. It sounds like he added a distortion pedal in the 

second half of the intro just before Bill & Tony joined in. 

Good luck. 

There is nothing special about the basic guitar sound - just moderate overdrive.

The LP recording has some effects and extra processing added, possibly out of phase reverb that provides something like echoes from the sides of the stereo image.

Either that or it's multi-tracked with another guitar track added.

Listen to a version from live video that's not been got-at and overdubbed with studio music; no studio effects, but purely what is really being played.

It's very basic overdrive, plus decades of technique and style.

eg. search 

"BLACK SABBATH - "N.I.B." from The End (Live Video)"

on youtube for a superb example.

You could use every same exact piece of equipment and not get the exact sound. Far more goes into sound than the equipment. You would need the same exact studio acoustics, the same relative humidity in the studio when the recording was done, and the same exact mix - and assuming everything else was identical, you might never get the same exact mix.